U.S. patent application number 12/167609 was filed with the patent office on 2009-01-08 for method for controlling the overpressure in a fuel-supply system of a common-rail type.
This patent application is currently assigned to Magneti Marelli Powertrain S. p. A.. Invention is credited to Francesco Paolo Ausiello, Matteo De Cesare, Gabriele Serra.
Application Number | 20090007885 12/167609 |
Document ID | / |
Family ID | 38924019 |
Filed Date | 2009-01-08 |
United States Patent
Application |
20090007885 |
Kind Code |
A1 |
Serra; Gabriele ; et
al. |
January 8, 2009 |
METHOD FOR CONTROLLING THE OVERPRESSURE IN A FUEL-SUPPLY SYSTEM OF
A COMMON-RAIL TYPE
Abstract
A method for controlling the overpressure in a fuel-supply
system of a common-rail type for an internal-combustion engine
provided with a number of cylinders; the method has the steps of
supplying fuel under pressure to a common rail connected to a
number of injectors by means of a high-pressure pump; detecting the
effective value of the pressure of the fuel within the common rail;
comparing the effective value of the pressure of the fuel within
the common rail with a safety value; determining a condition of
emergency if the effective value of the pressure of the fuel within
the common rail is higher than the safety value; and driving, in
the case of emergency, the injectors for discharging part of the
fuel present in the common rail so as to contain the increase in
pressure of the fuel within the common rail.
Inventors: |
Serra; Gabriele; (San
Lazzaro di Savena, IT) ; De Cesare; Matteo;
(Torremaggiore, IT) ; Ausiello; Francesco Paolo;
(Bologna, IT) |
Correspondence
Address: |
KLARQUIST SPARKMAN, LLP
121 SW SALMON STREET, SUITE 1600
PORTLAND
OR
97204
US
|
Assignee: |
Magneti Marelli Powertrain S. p.
A.
|
Family ID: |
38924019 |
Appl. No.: |
12/167609 |
Filed: |
July 3, 2008 |
Current U.S.
Class: |
123/456 ;
123/457 |
Current CPC
Class: |
F02M 47/027 20130101;
F02M 63/0215 20130101; F02M 2200/18 20130101; F02M 63/0235
20130101 |
Class at
Publication: |
123/456 ;
123/457 |
International
Class: |
F02M 69/46 20060101
F02M069/46 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 5, 2007 |
EP |
07425416.0 |
Claims
1. A method for controlling the overpressure in a fuel-supply
system of a common-rail type for an internal-combustion engine
provided with a number of cylinders; the method comprising:
supplying fuel under pressure to a common rail connected to a
number of injectors by means of a high-pressure pump; detecting the
effective value of the pressure of the fuel within the common rail;
comparing the effective value of the pressure of the fuel within
the common rail with a safety value of the pressure of the fuel
within the common rail; and determining a condition of emergency if
the effective value of the pressure of the fuel within the common
rail is higher than the safety value of the pressure of the fuel
within the common rail; said method being characterized in that it
comprises the further step of driving, in the case of emergency,
the injectors for discharging part of the fuel present in the
common rail so as to contain the increase in the pressure of the
fuel within the common rail.
2. The method according to claim 1, wherein the high-pressure pump
receives the fuel from a low-pressure pump, in the case of
emergency, there being envisaged the further step of turning off
the low-pressure pump.
3. The method according to claim 1, wherein, in the case of
emergency, the injectors are driven for increasing the flow rate
(m.sub.Inj) of fuel injected into the cylinders with respect to the
flow rate necessary for generation of the torque required by the
engine control.
4. The method according to claim 3, wherein, in the case of
emergency, supplementary openings of the injectors are made when
said supplementary openings do not give rise to combustion and
hence to delivery of undesired torque.
5. The method according to claim 4, wherein the supplementary
openings of the injectors are made during the step of exhaust of
the cylinders and during the terminal part of the step of expansion
of the cylinders.
6. The method according to claim 3 and comprising the further step
of reducing, in the case of emergency, the flow rate of air taken
in by the cylinders.
7. The method according to claim 1, wherein the injectors have a
hydraulic actuation of the needle and absorb for their actuation a
certain flow rate (m.sub.BackFlow) of fuel, which is discharged
into an exhaust channel; in the case of emergency, the injectors
being driven for increasing the flow rate (m.sub.BackFlow) of fuel
absorbed by the injectors themselves for their actuation and
discharged into the exhaust channel.
8. The method according to claim 7, wherein, in the case of
emergency, the injectors are preferably driven only for increasing
the flow rate (m.sub.BackFlow) of fuel absorbed by the injectors
themselves for their actuation and, only in the case of need, are
also driven for increasing the flow rate (m.sub.Inj) of fuel
injected into the cylinders with respect to the flow rate necessary
for the generation of the torque required by the engine
control.
9. The method according to claim 7 and comprising the further steps
of: determining for the injectors a threshold value (ET.sub.min) so
that each injector does not make any injection of fuel if it is
driven for a time interval shorter than the threshold value
(ET.sub.min); and increasing, in the case of emergency, the flow
rate (m.sub.BackFlow) of fuel absorbed by the injectors for their
actuation by driving the injectors themselves for a driving time
interval (ET.sub.red) shorter than the threshold value (ET.sub.min)
when the injectors themselves are not used for the injection of the
fuel required by the process of combustion.
10. The method according to claim 1, wherein the condition of
emergency is determined even when a malfunctioning of a pressure
sensor that measures the pressure of the fuel within the common
rail is detected.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims the benefit of European Patent
Application No. 07425416.0 filed Jul. 5, 2007, the entire contents
of which hereby are incorporated by reference.
TECHNICAL FIELD
[0002] The present invention relates to a method for controlling
the overpressure in a fuel-supply system of a common-rail type.
BACKGROUND ART
[0003] In current systems for direct injection of fuel of a
common-rail type, a low-pressure pump supplies the fuel from a tank
to a high-pressure pump, which in turn supplies the fuel to a
common channel or "common rail". Connected to the common rail are a
series of injectors (one for each cylinder of the engine), which
are cyclically driven so as to inject part of the fuel under
pressure present in the common rail within the respective
cylinders. For proper operation of combustion, it is important that
the value of the pressure of the fuel within the common rail should
always be kept at a desired value, which may generally vary as a
function of the engine point.
[0004] In order to keep the value of the pressure of the fuel
within the common rail at the desired value, it has been proposed
to size the high-pressure pump to supply the common rail with an
amount of fuel exceeding the effective consumption in every
condition of operation. Coupled to the common rail is an
electromechanical pressure regulator, which keeps the value of the
pressure of the fuel within the common rail at the desired value by
discharging the fuel in excess to a recirculation channel that
re-introduces said excess fuel upstream of the low-pressure pump.
An injection system of this type presents different drawbacks, in
so far as the high-pressure pump must be sized for supplying to the
common rail an amount of fuel that is slightly in excess of the
maximum possible consumption. However, said condition of maximum
possible consumption occurs somewhat rarely and in all the
remaining conditions of operation the amount of fuel supplied to
the common rail by the high-pressure pump is much greater than the
actual consumption, and hence a considerable portion of said fuel
must be discharged by the pressure regulator into the recirculation
channel. The work performed by the high-pressure pump to pump the
fuel that is subsequently discharged by the pressure regulator is
"useless" work. Hence, this injection system presents a very low
energy efficiency. Furthermore, this injection system tends to
overheat the fuel, in so far as, when the fuel in excess is
discharged by the pressure regulator into the recirculation
channel, the fuel itself passes from a very high pressure to a
substantially ambient pressure and, as a result of said pressure
jump, heats up.
[0005] In order to solve the problems described above, it has been
proposed to use a high-pressure pump with variable capacity capable
of supplying the common rail only with the amount of fuel necessary
for keeping the pressure of the fuel within the common rail at the
desired value.
[0006] For example, the patent application No. EP0481964A1
describes a high-pressure pump provided with an electromagnetic
actuator, which is able to vary instant by instant the capacity of
the high-pressure pump by varying the instant of closing of an
intake valve of the high-pressure pump itself In other words, the
capacity of the high-pressure pump is varied by varying the instant
of closing of the intake valve of the high-pressure pump itself. In
particular, the capacity is decreased by delaying the instant of
closing of the intake valve and is increased by anticipating the
instant of closing of the intake valve.
[0007] A further example of a high-pressure pump with variable
capacity is provided by the U.S. Pat. No. 6,116,870A1. The
high-pressure pump described in U.S. Pat. No. 6,116,870A1 comprises
a cylinder provided with a piston having a reciprocating motion
within the cylinder, an intake channel, a delivery channel
connected to the common rail, an intake valve designed to enable
passage of a flow of fuel entering the cylinder, a unidirectional
delivery valve coupled to the delivery channel and designed to
enable just a flow of fuel out of the cylinder, and a regulation
device coupled to the intake valve to keep the intake valve open
during a step of compression of the piston and hence enable a flow
of fuel from the cylinder through the intake channel. The intake
valve comprises a valve body that can move along the intake channel
and a valve seat, which is designed to be engaged in a fluid-tight
way by the valve body and is set at the end of the intake channel
opposite to the end communicating with the cylinder. The regulation
device comprises a control element, which is coupled to the valve
body and is mobile between a passive position, in which it allows
the valve body to engage in a fluid-tight way the valve seat, and
an active position, in which it does not allow the valve body to
engage the valve seat in a fluid-tight way. Coupled to the control
element is an electromagnetic actuator, which is designed to
displace the control element between the passive position and the
active position.
[0008] In the case of (mechanical, electrical or electronic)
malfunctioning of the variable-capacity high-pressure pump, the
variable-capacity high-pressure pump itself could supply the common
rail with an amount of fuel much higher than the necessary amount,
thus causing a fast rise in the pressure of the fuel within the
common rail. Once said situation of malfunctioning of the
high-pressure pump has been detected, the low-pressure pump is
immediately turned off in order to interrupt flow of fuel to the
high-pressure pump and hence block the uncontrolled increase in the
pressure of the fuel within the common rail. However, turning-off
of the low-pressure pump has effect with a certain delay (equal to
a certain number of pumping cycles of the high-pressure pump), and
hence, without any further interventions of limitation, the
pressure of the fuel within the common rail could reach values
higher than the maximum value that can be physically withstood by
the components of the injection system, with consequent failure of
said components and outflow of fuel at a high pressure into the
engine compartment. In order to limit the maximum pressure of the
fuel within the common rail in the event of malfunctioning of the
high-pressure pump, in known injection systems there is always
present an electromechanical pressure regulator controlled by a
control unit or else a mechanical pressure limiter.
[0009] However, coupling of an electromechanical pressure regulator
or a mechanical pressure limiter to the common rail with the
corresponding pipes for relief into the tank entails a
non-negligible cost both in terms of purchase of the components and
in terms of installation of said components; said cost is far from
justified by the sporadic nature of the cases of intervention
(i.e., cases of malfunctioning of the high-pressure pump that cause
a sudden increase in the pressure of the fuel within the common
rail).
DISCLOSURE OF INVENTION
[0010] The aim of the present invention is to provide a method for
controlling the overpressure in a fuel-supply system of a
common-rail type, said control method being free from the drawbacks
described above and, in particular, being easy and inexpensive to
implement.
[0011] Provided according to the present invention is a method for
controlling the overpressure in a fuel-supply system of a
common-rail type according to what is recited in the attached
claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0012] The present invention will now be described with reference
to the annexed drawings, which illustrate a non-limiting example of
embodiment thereof, wherein:
[0013] FIG. 1 is a schematic view of a system for direct injection
of fuel of a common-rail type that implements the control method
forming the subject of the present invention;
[0014] FIG. 2 is a schematic view, in side elevation and sectioned,
of a fuel injector of the system for direct injection of fuel of
FIG. 1;
[0015] FIG. 3 is a view at an enlarged scale of a detail of FIG. 2;
and
[0016] FIG. 4 is a graph that shows schematically the time plot of
some quantities of the system for direct injection of fuel of FIG.
1 during a malfunctioning of a high-pressure pump.
PREFERRED EMBODIMENTS OF THE INVENTION
[0017] In FIG. 1, the reference number 1 designates as a whole a
system of a common-rail type for direct injection of fuel into an
internal-combustion engine 2 provided with four cylinders 3. The
injection system 1 comprises four injectors 4, each of which is
designed to inject the fuel directly within a respective cylinder 3
of the engine 2 and receives the fuel under pressure from a common
rail 5.
[0018] A high-pressure pump 6 supplies fuel to the common rail 5 by
means of a pipe 7 and is provided with a device 8 for regulating
the flow rate, said device being governed by a control unit 9,
designed to keep the pressure of the fuel within the common rail 5
at a desired value, which generally varies in time as a function of
the engine point (i.e., of the conditions of operation of the
engine 2). By way of example, the regulation device 8 comprises an
electromagnetic actuator (not illustrated), which is able to vary
instant by instant the flow rate m.sub.HP of fuel of the
high-pressure pump 6 by varying the instant of closing of an intake
valve (not illustrated) of the high-pressure pump 6 itself. In
particular, the flow rate m.sub.HP of fuel is decreased by delaying
the instant of closing of the intake valve (not illustrated) and is
increased by anticipating the instant of closing of the intake
valve (not illustrated).
[0019] A low-pressure pump 10 with substantially constant capacity
supplies the fuel from a tank 11 to the high-pressure pump 6 by
means of a pipe 12.
[0020] The control unit 9 regulates the flow rate m.sub.HP of fuel
of the high-pressure pump 6 by means of a feedback control using as
feedback variable the value of the pressure of the fuel within the
common rail 5, said pressure value being detected in real time by a
sensor 13.
[0021] Each injector 4 is governed cyclically by the control unit 9
so that it will inject the fuel into a respective cylinder 3 of the
engine. The injectors 4 have a hydraulic actuation of the needle
and are hence connected to an exhaust channel 14, which has a
pressure that is a little higher than the ambient pressure and
which gives out upstream of the low-pressure pump 10, typically
inside the tank 11.
[0022] According to what is illustrated in FIGS. 2 and 3, each
injector 4 of fuel is housed in a cylindrical body 15 having a
longitudinal axis 16 and is governed so as to inject fuel from an
injection nozzle 17 regulated by an injection valve 18. Made within
the cylindrical body 15 is an injection chamber 19, which is
delimited at the bottom by a valve seat 20 of the injection valve
18 and houses in a slidable way a bottom portion of a needle 21 of
the injection valve 18, in such a way that the needle 21 will be
able to displace along the longitudinal axis 16 under the thrust of
a hydraulic actuator device 22 between a position of closing and a
position of opening of the valve seat 20.
[0023] A top portion of the needle 21 is housed in a control
chamber 23 and is coupled to a spring 24, which exerts on the
needle 21 itself a force directed downwards that tends to keep the
needle 21 itself in the closing position.
[0024] The cylindrical body 15 moreover has a supply channel 25,
which starts from a top end of the cylindrical body 15 and supplies
the fuel under pressure to the injection chamber 19. Branching off
from the supply channel 25 is a further supply channel 26, which is
designed to set the supply channel 25 in communication with the
control chamber 23 for supplying the fuel under pressure also to
the control chamber 23.
[0025] Starting from the control chamber 23 is an exhaust pipe 27,
which gives out into a top portion of the cylindrical body 15 and
sets the control chamber 23 in communication with the exhaust
channel 14. The exhaust pipe 27 is regulated by a control valve 28,
which is set in the proximity of the control chamber 23 and is
controlled by an electromagnetic actuator 29 between a closing
position, in which the control chamber 23 is isolated from the
exhaust pipe 27, and an opening position, in which the control
chamber 23 is connected to the exhaust pipe 27. The electromagnetic
actuator 29 comprises a spring 30, which tends to keep the control
valve 28 in the closing position.
[0026] The section of the supply channel 26, the section of the
control valve 28, and the section of the exhaust pipe 27 are sized
with respect to the section of the supply channel 25 in such a way
that, when the control valve 28 is open, the pressure of the fuel
in the control chamber 23 will drop to much lower values as
compared to the pressure of the fuel in the injection chamber 19
and in such a way that the flow rate of fuel that flows through the
exhaust pipe 27 is a fraction of the flow rate of fuel that flows
through the injection nozzle 17.
[0027] In use, when the electromagnetic actuator 29 is
de-energized, the force generated by the spring 30 keeps the
control valve 28 in the closing position. Thus, the pressure of the
fuel in the control chamber 23 is the same as the pressure of the
fuel in the injection chamber 19 as a result of the supply channel
26. In this situation, the force generated by the spring 24 and the
hydraulic force generated by the imbalance of the useful areas of
the needle 21, to the advantage of the control chamber 23 with
respect to the injection chamber 19, keep the injection valve 18 in
the closing position.
[0028] When the electromagnetic actuator 29 is energized, the
control valve 28 is brought into the opening position against the
force of the spring 30. Hence the control chamber 23 is set in
communication with the exhaust channel 14, and the pressure of the
fuel in the control chamber 23 drops to much lower values as
compared to the pressure of the fuel in the injection chamber 19.
As has been said previously, the difference between the pressures
of the fuel in the injection chamber 19 and in the control chamber
23 is due to the sizing of the sections of the supply channel 26,
of the control valve 28, and of the exhaust pipe 27 with respect to
the section of the supply channel 25.
[0029] As a result of the imbalance between the pressures of the
fuel in the injection chamber 19 and in the control chamber 23, on
the needle 21 a hydraulic force is generated, which displaces the
needle 21 upwards against the action of the spring 24 so as to
bring the injection valve 18 into the opening position and enable
injection of the fuel through the injection nozzle 17.
[0030] When the electromagnetic actuator 29 is de-energized, the
force generated by the spring 30 brings the control valve 28 into
the closing position. Hence, the pressure of the fuel in the
control chamber 23 tends to rise until it reaches the pressure of
the fuel in the injection chamber 19. In this situation, the force
generated by the spring 24 and the hydraulic force generated by the
imbalance of the useful areas of the needle 21, to the advantage of
the control chamber 23 with respect to the injection chamber 19,
bring the injection valve 18 into the aforementioned closing
position.
[0031] Preferably, the supply channel 26 has a restricted portion
to obtain an instantaneous increase in the difference of pressure
between the control chamber 23 and the injection chamber 19 during
the transient of closing of the needle 21 (i.e., when the needle 21
passes from the opening position to the closing position) so as to
increase the force acting on the needle 21 and, hence, speed up
closing of the needle 21 itself.
[0032] From what has been set forth above, it is clear that, when
the electromagnetic actuator 29 of an injector 4 is controlled,
initially the control valve 28 is opened, and the fuel present in
the control chamber 23 starts to flow through the exhaust pipe 27
and towards the exhaust channel 14. After a certain time interval
from opening of the control valve 28, on the needle 21 a force of
thrust of a hydraulic nature is generated, which causes opening of
the injection valve 18 and hence supply of fuel through the
injection nozzle 17.
[0033] In other words, the supply of fuel through the injection
nozzle 17 occurs only if the electromagnetic actuator 29 of an
injector 4 is controlled for a time interval longer than a
threshold value ET.sub.min. Instead, if the electromagnetic
actuator 29 of an injector 4 is controlled for a time interval
lower than the threshold value ET.sub.min, then there may occur
opening of the control valve 28 and consequent outflow of fuel to
the exhaust channel 14, but no supply of fuel through the injection
nozzle 17 occurs. Obviously, if the electromagnetic actuator 29 of
an injector 4 is controlled for a time interval that is extremely
short and much shorter than the threshold value ET.sub.min, then
not even opening of the control valve 28 occurs.
[0034] The threshold value ET.sub.min of an injector 4 is linked to
the characteristics, tolerances, and ageing of the components of
the injector 4 itself. Consequently, the threshold value ET.sub.min
can vary (slightly) from injector 4 to injector 4 and, for one and
the same injector 4, can also vary (slightly) during the life of
the injector 4 itself. Furthermore, the threshold value ET.sub.min
of an injector 4 can vary in a way inversely proportional also to
the value of the pressure of the fuel in the common rail 5, i.e.,
the higher the pressure of the fuel in the common rail 5, the lower
the threshold value ET.sub.min.
[0035] With reference to FIG. 1, the control unit 9 determines
instant by instant a desired value of the pressure of the fuel
within the common rail 5 as a function of the engine point and
consequently acts in order for the effective value of the pressure
of the fuel within the common rail 5 to follow the desired value
rapidly and precisely.
[0036] The variation dP/dt of the pressure of the fuel within the
common rail 5 is given by the following state equation of the
common rail 5:
dP/dt=(k.sub.b/Vr).times.(m.sub.HP-m.sub.Inj-m.sub.Leak-m.sub.BackFlow)
[1]
where [0037] dP/dt is the variation of the pressure of the fuel
within the common rail 5; [0038] k.sub.b is the bulk modulus of the
fuel; [0039] V.sub.r is the volume of the common rail 5; [0040]
m.sub.HP is the flow rate of fuel of the high-pressure pump 6;
[0041] m.sub.Inj is the flow rate of fuel injected into the
cylinders 3 by the injectors 4; [0042] m.sub.Leak is the flow rate
of fuel lost by leakage from the injectors 4; [0043] m.sub.BackFlow
is the flow rate of fuel absorbed by the injectors 4 for their
actuation and discharged into the exhaust channel 14.
[0044] From the above equation, it emerges clearly that the
variation dP/dt of the pressure of the fuel within the common rail
5 is positive if the flow rate m.sub.HP of fuel of the
high-pressure pump 6 is greater than the sum of the flow rate
m.sub.Inj of fuel injected into the cylinders 3 by the injectors 4,
of the flow rate m.sub.Leak of fuel lost owing to leakage from the
injectors 4, and of the flow rate m.sub.BackFlow of fuel absorbed
by the injectors 4 for their actuation and discharged into the
exhaust channel 14. It should be noted that the flow rate m.sub.Inj
of fuel injected into the cylinders 3 by the injectors 4 and the
flow rate m.sub.BackFlow of fuel absorbed by the injectors 4 for
their actuation and discharged into the exhaust channel 14 are
extremely variable (they can even be zero) according to the
modalities of control of the injectors 4, whereas the flow rate
m.sub.Leak of fuel lost owing to leakage from the injectors 4 is
quite constant (it presents only a slight increase as the pressure
of the fuel within the common rail 5 increases) and is always
present (i.e., it is never zero).
[0045] When the control unit 9 detects a condition of emergency,
i.e., the presence of malfunctioning of the high-pressure pump 6,
which causes a sudden increase in the pressure of the fuel within
the common rail 5 (for example, said control unit 9 detects, by
means of the pressure sensor 13, an unexpected and sudden increase
of the pressure of the fuel in the common rail 5), the control unit
9 itself turns off the low-pressure pump 10 immediately to stop
supply of the high-pressure pump 6 (i.e., to interrupt the flow of
fuel to the high-pressure pump 6). Furthermore, in order to prevent
the pressure of the fuel within the common rail 5 from exceeding a
safety value that guarantees tightness and integrity of the
injection system 1, the control unit 9 governs the injectors 4
(i.e., it energizes the electromagnetic actuators 29 of the
injectors 4) to discharge part of the fuel present in the common
rail 5, i.e., to increase the flow rate m.sub.BackFlow of fuel
absorbed by the injectors 4 for their actuation and discharged into
the exhaust channel 14 and possibly also to increase the flow rate
m.sub.Inj of fuel injected into the cylinders 3 by the injectors 4
as compared to the flow rate necessary for generation of the torque
required by the engine control.
[0046] In other words, according to the increase in pressure of the
fuel present in the common rail 5, the control unit 9 decides
whether in order to contain said increase it is sufficient to
increase the flow rate m.sub.BackFlow of fuel absorbed by the
injectors 4 for their actuation and discharged into the exhaust
channel 14 or else whether it is necessary also to increase the
flow rate m.sub.Inj of fuel injected into the cylinders 3 by the
injectors 4 with respect to the flow rate necessary for generation
of the torque required by the engine control. Obviously, the higher
the increase in pressure of the fuel present in the common rail 5
(i.e., the higher the flow rate m.sub.HP of fuel of the
high-pressure pump 6 is than the actual needs), the more likely it
is that, in order to contain said increase, the control unit 9 will
also have to increase the flow rate m.sub.Inj of fuel injected into
the cylinders 3 by the injectors 4 with respect to the flow rate
necessary for generation of the torque required by the engine
control.
[0047] In order to increase the flow rate m.sub.BackFlow of fuel
absorbed by the injectors 4 for their actuation and discharged into
the exhaust channel 14, the control unit 9 drives the injectors 4
(i.e., it energizes the electromagnetic actuators 29 of the
injectors 4) with a train of pulses, each of which has a driving
time interval ET.sub.red close to, but shorter than, the respective
threshold values ET.sub.min when the injectors 4 themselves are not
used for injection of the fuel required by the process of
combustion. In this way, no injection of fuel into the cylinders 3
is made, but the flow rate m.sub.BackFlow of fuel absorbed by the
injectors 4 for their actuation and discharged into the exhaust
channel 14 is increased. It should be emphasized that the driving
time interval ET.sub.red with which each injector 4 is driven must
be shorter than the threshold value ET.sub.min, but must not be
excessively shorter than the threshold value ET.sub.min. Otherwise,
the amount of fuel that is discharged into the exhaust channel 14
is far from significant and even zero. In other words, said control
strategy envisages a series of micro-actuations of the injectors 4
when the injectors 4 themselves are not used for injection of the
fuel required by the combustion process.
[0048] The duration of the driving time interval ET.sub.red of each
injector 4 generally depends upon the pressure of the fuel within
the common rail 5 and must always be shorter than the threshold
value ET.sub.min in order to prevent undesirable fuel injection
within the cylinders 3. Since, as has been said previously, the
threshold value ET.sub.min can vary from injector 4 to injector 4
as well as during the life of a given injector 4, it is preferable
to implement in the control unit 9 an algorithm of optimization of
the duration of the driving time interval ET.sub.red of each
injector 4 in order to prevent said driving time interval
ET.sub.red from possibly exceeding the threshold value
ET.sub.min.
[0049] In order to increase the flow rate m.sub.Inj of fuel
injected into the cylinders 3 by the injectors 4 with respect to
the flow rate necessary for generation of the torque required by
the engine control, the control unit 9 carries out supplementary
openings of the injectors 4 preferably when said supplementary
openings do not give rise to any combustion and hence to any
delivery of undesired torque. For example, the control unit 9 could
perform the supplementary openings of the injectors 4 only during
the step of exhaust of the cylinders 3 (or also during the terminal
part of the expansion step). In fact, during the step of exhaust of
each cylinder 3 the fuel that is injected into the cylinder 3
itself does not burn (hence, it does not cause any generation of
undesired torque) and is immediately expelled into the exhaust
system.
[0050] In particularly critical situations (typically when
malfunctioning of the high-pressure pump 6 arises during a cut-off
step in which the flow rate m.sub.Inj of fuel injected into the
cylinders 3 by the injectors 4 is normally zero), in order to limit
adequately the increase in the pressure of the fuel present in the
common rail 5, it might not be sufficient to perform supplementary
openings of the injectors 4 only when said supplementary openings
do not give rise to any combustion and hence to delivery of
undesired torque. In this case, it may be useful to reduce (by
appropriately controlling the throttle valve that regulates the
flow rate of intake air) the flow of air taken in by the cylinders
3 in such a way as to prevent in any case combustion of the
supplementary fuel injected into the cylinders 3 during the
supplementary openings on account of lack of combustion air.
[0051] It should be noted that the reduction in the flow rate of
air taken in by the cylinders 3 is useful not only for preventing,
on account of lack of combustion air, combustion of the
supplementary fuel within the cylinders 3, but also for preventing,
on account of lack of combustion air, combustion of the
supplementary fuel within the exhaust system. In this way, it is
possible to prevent an excessive overtemperature in the exhaust
system that could damage the exhaust system itself.
[0052] To sum up what has been described above, when the control
unit 9 detects an unexpected and sudden increase in the pressure of
the fuel in the common rail 5, the control unit 9 itself
immediately turns off the low-pressure pump 10 to stop supply to
the high-pressure pump 6. Furthermore, in order to prevent the
pressure of the fuel within the common rail 5 from exceeding a
safety value that guarantees tightness and integrity of the
injection system 1, the control unit 9 drives the injectors 4 for
discharging part of the fuel present in the common rail 5 by
imparting on the injectors 4 a burst of micro-actuations that will
be able to increase the flow rate m.sub.BackFlow of fuel absorbed
by the injectors 4 for their actuation and possibly by carrying out
supplementary openings of the injectors 4 preferably during the
step of exhaust of the cylinders 3. If the control unit 9 carries
out supplementary openings of the injectors 4, then the control
unit 9 itself closes the throttle valve that regulates the flow
rate of intake air so as to reduce the flow rate of air taken in by
the cylinders 3 in such a way as to prevent in any case combustion
of the supplementary fuel injected into the cylinders 3 during the
supplementary openings on account of lack of combustion air.
[0053] What has been set forth above is represented schematically
in the graph of FIG. 4, where at the instant t.sub.1 the
high-pressure pump 6 presents malfunctioning, which causes an
irregular increase in the flow rate m.sub.HP of fuel of the
high-pressure pump 6. In FIG. 4, m.sub.HP designates the expected
flow rate of fuel of the high-pressure pump 6, whilst M.sub.Phil is
the effective flow rate of fuel of the high-pressure pump 6.
Following upon malfunctioning of the high-pressure pump 6, the
pressure of the fuel in the common rail 5 (designated by p in FIG.
4) increases from a value p.sub.1, which is the desired working
value, until it reaches a value p.sub.2, which is the intervention
threshold of the emergency procedure described above. When the
pressure of the fuel in the common rail 5 reaches the value
p.sub.2, which is the intervention threshold of the emergency
procedure described above, the control unit 9 turns the
low-pressure pump 10 off (m.sub.LP is the flow rate of fuel of the
low-pressure pump 10) and drives the injectors 4 in order to
increase the flow rate m.sub.BackFlow of fuel absorbed by the
injectors 4 for their actuation and discharged into the exhaust
channel 14 and to increase the flow rate m.sub.Inj of fuel injected
into the cylinders 3 by the injectors 4. In FIG. 4 rpm is the
r.p.m. of the engine 2.
[0054] As has been said previously, the control unit 9 intervenes
by turning off the low-pressure pump 10 and limiting the pressure
of the fuel within the common rail 5 when it detects the presence
of malfunctioning of the high-pressure pump 6, which causes a
sudden increase in the pressure of the fuel within the common rail
5 itself. A similar intervention is made by the control unit 9 also
when the control unit 9 itself detects malfunctioning of the
pressure sensor 13, which makes it impossible to know with adequate
precision the pressure of the fuel within the common rail 5.
[0055] The control strategy described above for managing an
emergency situation linked to malfunctioning of the high-pressure
pump 6 presents the advantage of being particularly effective in
containing the increase in the pressure of the fuel in the common
rail 5, at the same time being extremely inexpensive to implement
in so far as it uses only components normally present in a modem
engine with direct injection of the fuel. In other words, it is no
longer necessary to associate to the common rail 5 an
electromechanical pressure regulator or a mechanical pressure
limiter for limiting the pressure of the fuel in the common rail 5
in the case of emergency in so far as said limitation is obtained
with the same degree of effectiveness by means of the control of
the injectors 4 described above.
* * * * *